The present invention relates to the field of optical wavelength filters and their inscription method into optical waveguides, and more particularly concerns a tunable phase mask assembly used for such an operation.
Optical filters usually consist of a periodic index change permanently photo-written into an optical waveguide, in order to create wavelength-selective mode coupling in this waveguide. The photo-writing process involves exposing the waveguide to an ultraviolet optical beam patterned as the desired periodic index change. Fiber Bragg Gratings (FBG) are wavelength-selective all-fiber filters photo-written by using an ultra-violet beam incident on a phase mask before exposing the fiber. FBGs are key components for the development of DWDM (Dense-Wavelength Division Multiplexing) optical communication networks.
The phase mask FBG writing technique is a well established reliable industrial process for the mass production of FBGs. In this approach, a phase mask having periodical grating corrugations is placed in the path of a light beam, which is diffracted by the mask to generate an interference pattern. This interference pattern is photoinduced in the fiber or other photosensitive material to write the FBG. However, an important drawback of the phase mask technique is that you can only write one type of FBG at a very precise center wavelength with each set-up. If gratings at different wavelengths have to be written, the phase mask has to be changed within the fabrication set-up which is time consuming, and requires keeping a very large and expensive stock of phase masks.
Different techniques have been proposed in the past to be able to tune the center wavelength of photo-written FBGs without changing the phase mask element that is part of the writing set-up. One such technique, suggested by G. A. Ball et al. in xe2x80x9cCompression-tuned single frequency Bragg grating fiber laserxe2x80x9d, Optics Letters, vol 19, no 23 (1994) pp. 1979-1981, consists of stretching or compressing the fiber behind the phase mask. In these cases, the fiber grating is photo-written at the same wavelength as when the fiber is unstretched or uncompressed; however, once the fiber is relaxed to its initial state, the grating is tuned accordingly. Tunability as large as 0.5% has been shown by stretching the fiber and as large as 2.5% by compressing the fiber. However, in both cases, the fiber is mechanically weakened during the process, and the risks of breaking the fiber are important. Additionally, it is difficult to accurately determine the elasticity of the fiber in order to obtain a perfectly repeatable process. Compressing the fiber without bending it is also a very challenging demand.
U.S. Pat. No. 5,671,307 (LAUZON et al.) shows another approach for tuning the center wavelength of photo-written FBGs, without changing the phase mask element that is part of the writing set-up. LAUZON teaches inducing a temperature change in the fiber, through the thermo-optic and thermal expansion coefficients of glass. However, silica glass (the material from which standard optical fibers are made) having very poor thermal conductivity, it is difficult to implement such temperature tuning over a fiber section of more than a few millimeters. Also, tunability of only 0.01 nm/xc2x0 C. can be expected using this method. Thus very high temperature changes of the order of 100xc2x0 C. would need to be used to have a substantial impact on the resulting FBG. Generating such high temperature changes is not cost and power efficient.
There is therefore a need for tuning the period of a FBG written using the phase mask technique without constantly changing the phase mask, or risking damages to the fiber.
It is therefore an object of the present invention to provide a phase mask having a tunable periodicity.
Accordingly, the present invention provides a tunable phase mask assembly for diffracting a light beam passing therethrough and generating an interference pattern having a period.
The phase mask assembly first includes a phase mask having a length extending between first and second opposed longitudinal ends. The phase mask also has a longitudinal surface provided with a plurality of grating corrugations projecting therefrom, distributed with a periodicity along the length of the phase mask.
The assembly also includes means for reversibly changing the length of the phase mask between the longitudinal ends thereof. In this manner, the periodicity of the grating corrugations is changed and the period of the interference pattern is tuned.
The present invention also provides a method for tuning a period of an interference pattern generated by a light beam diffracted by a phase mask. The phase mask has a length extending between two opposed longitudinal ends, and a longitudinal surface provided with a plurality of grating corrugations projecting therefrom. The grating corrugations are distributed with a periodicity along the length of the phase mask. The method includes a step of reversibly changing the length of the phase mask between the longitudinal ends thereof, thereby changing the periodicity of the grating corrugations and tuning the period of the interference pattern.
In accordance with preferred embodiments of the present invention, it is the phase mask, not the optical fiber, that is compressed or stretched in order to change the center wavelength of the photo-written FBG. The phase mask corrugation period translates directly, through a mathematical formula that varies with the phase mask characteristics, into the light beam periodicity that in turn translates directly into the index change grating period and thus on the wavelength response of the photo-written FBG. Stretching or compressing the phase mask has a direct impact on its corrugation period. Advantageously, using phase masks that can be compressed or stretched results in a tunable fabrication method for FBGs, that is a non-contact method for the FBG, it thus has no impact on its post-FBG writing mechanical performance of the fiber. It ensures good reproducibility and good performance since what you see (during the FBG writing process) is what you get. Also, performance is ensured in an industrial production situation because it is much easier to have a good grip or do a controlled compression on a substrate having a regular shape with large contact surfaces in which the phase mask is imprinted, than on the very small and flexible optical fiber.
Further features and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.